Changing messaging bundle?

I have T-Mobile. My bill ends on May 5th. Also, I qualify for an upgrade too but I don't think it has anything to do with this. I currently have the 400 texting package. I used 321 out of 400. I'm wondering if I could change to unlimited for the family right now. Will I be charged with $5 (400 texing package) and $19.99 (if I change to unlimited for family) or will I be charged with the messages I used plus the unlimited? Thank you so much! I'm gonna be changing it online @the T-Mobile website.